This invention relates to an optical isolator which is an optical part used in an optical communication system and has a garnet crystal as a Faraday rotator.
In an optical communication system, when transmission light emitted from a semiconductor-laser light source is transmitted through an optical system, a communication obstacle will be brought about if part of the light is reflected by the entrance end face of the optical system and is returned to the light source. An optical isolator, provided to block such return light, is an optical part interposed between the laser and the optical system, and is such that a Faraday rotator constructed of a garnet crystal always assuming magnetism is sandwiched between a polarizer and an analyzer and is encased, for example, in a cylindrical magnet.
Since the optical isolator is used integrally with a semiconductor laser, it is necessary to satisfy requirements that: (1) it is small in size and can be mounted directly to the surface of the semiconductor laser, (2) the element length of the Faraday rotator is short, (3) the temperature dependency of an extinction ratio is small, and (4) a loss in the amount of incident light is slight. In order to miniaturize the optical isolator, an attempt is made to reduce the saturation magnetization of the Faraday rotator to thereby make a magnet small. For example, Japanese Patent Provisional Publication No. Hei 9-185027 discloses a bismuth-substitution rare-earth iron garnet crystal, as a Faraday rotator which is low in saturation magnetization and exhibits stable, rectangular hysteresis characteristics. However, this garnet crystal, because of its weak Faraday rotation effect, is such that the element length of the Faraday rotator is long and the temperature dependency of an angle of Faraday rotation is large. Japanese Patent Provisional Publication No. Hei 11-1394 discloses a low-saturation bismuth-substitution iron garnet single crystal film which is low in saturation magnetization, has a considerable Faraday rotation effect, and is small in temperature dependency of an angle of Faraday rotation. In this crystal, however, the element length of the Faraday rotator is long.
A small-sized, optical isolator satisfying the above requirements has not been available in the past.
It is, therefore, an object of the present invention to provide an optical isolator with a garnet crystal for a Faraday rotator which is small in size and can be mounted directly and in which the temperature dependency of an extinction ratio and a loss in the amount of incident light are small.
In order to achieve this object, the garnet crystal of the present invention is grown by a liquid-phase epitaxial growth technique on a substrate of a garnet with a lattice constant of 12.514xc2x10.015 xc3x85, and is expressed by the following composition formula:
(Tb1xe2x88x92(a+b+c+d)LnaBibM1cEud)3(Fe1xe2x88x92eM2e)5O12
In this formula, Ln is an element selected from rare-earth elements excluding Tb and Eu, and Y; M1 is an element selected from elements of Ca, Mg, and Sr; M2 is an element selected from elements of Al, Ga, Sc, In, Ti, Si, and Ge; a, b, c, d, and e are defined as 0xe2x89xa6axe2x89xa60.5, 0.3 less than bxe2x89xa60.6, 0xe2x89xa6cxe2x89xa60.02, 0 less than dxe2x89xa60.3, and 0.01 less than exe2x89xa60.3, respectively.
Bi serves to improve the Faraday rotation effect in proportion to its amount existing in the garnet crystal and to reduce the element length of the Faraday rotator. Bi must exist within the range of 0.3 less than bxe2x89xa60.6 in the garnet crystal, and thereby a Faraday rotation coefficient at a wavelength of 1.55 xcexcm band used in an ordinary optical communication system can be set to at least 1050xc2x0/cm. If the value of b exceeds 0.6, the lattice constant of the garnet crystal will be out of the desired range.
Ca, Mg, or Sr represented by the element M1 serves to improve the transmittance of light of the garnet crystal and to minimize a loss in the amount of incident light.
Eu is indispensably contained in the garnet crystal to improve the transmittance of light of the garnet crystal and to minimize a loss in the amount of incident light.
The element M2 is Al, Ga, Sc, In, Ti, Si, or Ge which can be replaced with Fe. When Al, Ga, Sc, or In is present, the lattice constant is set in the desired range, and saturation magnetization is reduced with respect to a saturation magnetic flux density. If Ti, Si, or Ge coexists with Fe which is a bivalent ion, in the garnet crystal, the transmittance of light of the garnet crystal will be improved, as in the case where Ca, Mg, or Sr coexists, and the loss in the amount of incident light will be minimized. The element M2 is contained within the range of 0.01 less than exe2x89xa60.3 in the garnet crystal. Beyond this range, the lattice constant is out of the desired range, and the absolute value of the Faraday rotation coefficient at a wavelength of 1.55 xcexcm band becomes 1000xc2x0/cm or less. This lessens the Faraday rotation effect.
It is desirable that the substrate of a garnet with a lattice constant of 12.514xc2x10.015 xc3x85 is formed of a garnet which does not contain Nd. As an example of such a substrate is cited NOG (a trade name in Shin-Etsu Chemical Co., Ltd.) which is the substrate of a garnet expressed by the chemical formula of (CaGdMgZrGa)8 O12 in which Ca, Zr, and Mg are added for substitution to a gadolinium-gallium garnet. Nd has the property of absorbing light with a wavelength of 1.55 xcexcm band. When a crystal is thus grown by the liquid-phase epitaxial growth technique on an NGG (a trade name in Shin-Etsu Chemical Co., Ltd.) substrate having a garnet structure expressed by the chemical formula of Nd3Ga5O12, Nd will be admixed into a growing garnet crystal if a small fraction of Nd3Ga5O12 is mixed with a molten material. This increases the loss in the amount of incident light on the Faraday rotator.
In this garnet crystal, a compensation temperature at which saturation magnetization becomes zero is xe2x88x9250xc2x0 C. or less and the saturation magnetization at temperatures of xe2x88x9240-100xc2x0 C. is as small as 400 gauss (31.8 KA/m) or less.
An optical isolator includes at least two parts of a Faraday rotator obtained from the above garnet crystal and an analyzer.
The optical isolator is generally used in the temperature range of xe2x88x9240-100xc2x0 C. The garnet crystal has the properties that the saturation magnetization is low in this temperature range, the compensation temperature at which the saturation magnetization becomes zero is lower than in the temperature range, and the Faraday rotation effect is heightened. The temperature dependency of the extinction ratio and the loss in the amount of incident light on the optical isolator with the Faraday rotator constructed of the garnet crystal are small.